Charles Lyell and ‘Modern changes of the Earth’: the Milledgeville Gully

One major emphasis of Lyellʹs Principles of Geology was on the abundance and efficacy of ‘present’ or ‘modern’ causes in producing geologic change. He was also determined that the evidence given for the activity of such causes should be scientific and reliable: to that end, he expended tremendous efforts to make his own observations. Perhaps inevitably, he stressed the operation of relatively high magnitude, low frequency events—storms, floods and earthquakes—especially those whose dates of onset were known. A key example of Lyellʹs interest in the ‘modern’ was his examination of a 20-year-old gully at Milledgeville, GA, in January 1846. He explicitly related its ultimate cause to the laying bare of the clayey surface horizons as a result of recent forest clearance and its proximate formation to sunbaked cracks in the clay and erosion by rainfall-induced runoff. He also saw this example as merely one of hundreds in the region. Over the 140 years since Lyellʹs visit, the gully has been repeatedly revisited and even surveyed (by H.A. Ireland in 1937). Its development shows continued growth, followed by increasing stabilisation. This seems to match evidence for the general behaviour of such landforms and the gully, therefore, may serve as a well-documented exemplar for the nature of localised accelerated erosion, regardless of the ultimate initiating factors. Lyellʹs interest in such short-term erosional features—as indicating the efficacy of modern causes—stands in contrast to his successorsʹ focus on ‘geological’ scale features such as Niagara Falls.